drm/i915: remove i915_inttoext_table debugfs entry

CHV hard hangs on reading these registers. As these have not
been used since cantiga & ilk, remove the debugfs entry.

References: https://bugs.freedesktop.org/show_bug.cgi?id=80893


Suggested-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>